Quote Originally Posted by SlowM90 View Post
Okay...Im soooo confused!!! Someone PLEASE clarify this! I was always told..the thicker the oil.. as in 10w-30 the better for colder area..as in up North...Not only was I told this but reading around I noticed almost everyone from up North runs 10W-30! And you would think you would want a thicker oil in the winter...not the summer. I have read up on this after reading this forum and some say heavier is better in the winter and some say noo its needed for the summer..So someone who knows can you please clarify this!
I looked around a couple sites and lifted this for you.

Most oils on the shelves today are "Multigrades", which simply means that the oil falls into 2 viscosity grades (i.e. 10w-40 etc)
Multigrades were first developed some 50 years ago to avoid the old routine of using a thinner oil in winter and a thicker oil in summer.
In a 10w-40 for example the 10w bit (W = winter, not weight or watt or anything else for that matter) simply means that the oil must have a certain maximum viscosity/flow at low temperature. The lower the "W" number the better the oils cold temperature/cold start performance.
The 40 in a 10w-40 simply means that the oil must fall within certain viscosity limits at 100 degC. This is a fixed limit and all oils that end in 40 must achieve these limits. Once again the lower the number the thinner the oil, a 30 oil is thinner than a 40 oil at 100 degC etc.

And just because this thread is really getting fun, I’ll throw my hat in the ring.

Castrol SYNTEC = driving in extreme climates (frigid winters/scorching summers)
Castrol GTX = driving in moderate climates
Redline = racers